Aquaculture Engineering

Unit 1 - Surveying

Principles of surveying
The fundamental principles of survey are based on two aspects.
1)Location of a point by measurement, from two points of references.
2) Working from whole to part.

Location of a point by measurement from two points of references
The relative positions of the points to be surveyed should be located by measurement from at least two points of reference, the positions of which have been already fixed.

Let P and Q be the reference points on the ground. The distance PQ can be measured accurately and the relative positions of P and Q can be plotted on the sheet to some scale, using these reference points other points can be located.

• The distance PQ is known, measure the distance QR and PR. The point R can beplotted by swinging the two arcs to the same scale to which PQ has been plotted. This principle is very much used in chain surveying.
• A perpendicular line can be dropped on the reference line PQ and the length PS and SR is measured. This principle is used for defining details.
• Using the distance QR and the angle PQR, the point R can be plotted. This principle is used in traversing.
• Using the angle PQR and RPQ the position of R can be plotted. This principle is very much used in triangulation and this method is used for very extensive work.
• Angle PQR and the distance PR can be used for plotting of R. This principle used in traversing is of minor utility.

Working from whole to part
It is very essential to establish first a system of control points and to fix them with higher precision. Minor control points can be established by less precise methods. The idea of working in this way is to prevent the accumulation of errors and to control and localize minor errors, which otherwise would expand to greater magnitude if the reverse process is followed thus making the work uncontrolled at the end.